Why some monkeys are better liars

Monkeys may see, hear and speak no evil, but they sure can be naughty, according to the first study to compare the ability of monkeys to deceive others in order to get food.

Intentional deceit is not restricted to humans, say Federica Amici and colleagues of Liverpool John Moores University in the UK. Some monkeys use simple forms of deceit, and the ability depends not on how closely related they are to humans, but on their social structure.

Amici’s team put up to 10 monkeys from three different primate species through the same experiment designed to test their ability to deceive dominant monkeys.

Secret stash

Spider monkeys, brown capuchins and long-tailed macaques were shown how to access food that was hidden or just out of reach. They were then put in cages with a socially higher-ranking monkey from the same species. Dominant monkeys in all three species would normally have priority over food, but in this case they did not know how to get to it.

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Subordinate monkeys of all three species went straight for the food when their dominant partner was not around. But as soon as the dominant monkey was introduced, they held back. This suggests they were intentionally withholding information in order to get the food for themselves.

The best deceivers were the macaques, which have a very strict social structure. Dominant individuals in macaque groups have very little tolerance for subordinates and claim priority to all resources for themselves.

Going hungry

“The point [of deceit] is to withhold information in a constructive way, to eventually get the food yourself,” says Filippo Aureli also of John Moores University.

However, if this is why the subordinate macaques didn’t access the hidden food in the presence of dominants, it didn’t work very well. “The subordinate long-tailed macaques almost never got the food,” says Aureli. In the absence of dominant individuals, subordinate macaques went straight to the hidden larders. But when their dominant partners were introduced into the cage they became so secretive they only rarely attempted to get the food for themselves.

For spider monkeys, the strategy worked. Subordinates waited until the dominant monkey was on the other side of the cage before going for the food and ultimately ate more than the subordinates of two other species.

Aureli and Amici believe this is because spider monkey social structure is both tolerant and fluid. Spider monkeys do not spend their entire lives with the same group. Groups frequently form, split into subgroups and reshuffle. This may make them more cunning, says Aureli. “When two subgroups merge, they need to reassess the information they have – who is around, who is not, and who the dominant individuals are.”

Aureli believes the results suggest that deceit is driven by social evolution. He points out that spider monkeys and humans are evolutionarily very distinct, but have similar flexible social structures. Deceit, he says, evolved independently in very distinct animal species and the common driver may well be how the animals organise themselves socially.

‘Not rocket science’

Richard Byrne, an evolutionary psychologist at the University of St Andrews, UK, points out that the researchers “very wisely” do not speculate about how the monkeys learned to deceive, and whether they can recognise that they know something that the other monkeys don’t – an ability known as “theory of mind”.

“Suppose in all the cases where they rushed in and grabbed the food they got beaten on the head and lost the food,” says Byrne. “It wouldn’t take a rocket scientist to work out that there are times when it’s best not to act. They might have learnt that it’s a good idea not to act without understanding why.”

The results may hint at the sort of society that theory of mind evolved in, though. They show that deceit is also more likely to pay off in a less dictatorial society. “This could mean that it’s more likely that the theory of mind light went on in a society full of complex social tactics,” says Byrne.